My 650G is measured from the bottom of the petal, forward to 'firewall', with tape parallel to the floor.
If steering is really hard (loads engine?) then the clutches are probably not disengaging before brakes engage. Have you done the 'immovable object' test in the manual? You should be able to disengage the clutch before the brake engages. If not, loosen the brakes. Or, adjust the clutches. The manual is quite good at explaining this. Since you are at 13" at the bottom, and normal should let the pedal drop to 11", yes, you probably need to loosen the brakes.
You may need to adjust the clutches. The manual on my 650G says clutches should start disengaging at 1" pedal movement.
I fought hard steering with mine for several days. Adjustment had supposedly been done at the dealer but later I found out an inexperienced mechanic had been sent to do the job. The dealer came out to readjust, improved but not fixed. So I got out my manual and followed it and now the 650G steers great.
Bob (still a newbee at 50 hours in the seat!)
Put a ruler on brake pedal and had 10 in from top and 13 in from bottom of brake pedal to fire wall. Book says 11 in when pushed in by hand and pedal is hard. I thought they might be too tight. I have to push very hard to turn tractor. Thanks